Thin-film magnetic transducers have very low impedance because of the relatively few turns in their winding. Accordingly, heavy current flow is necessary for successful writing, and the signals produced by the winding during readback are relatively small and susceptible to crosstalk and noise. Hence, it is desirable to provide an amplifier which has very low input impedance and which is located as close to the transducer as possible to make the lead lines from the winding as short as possible. This is explained, for example, in IBM Technical Disclosure Bulletin, Vol. 22, No. 1, June, 1979, p. 331, "Magnetic Head Assembly Including Head Circuitry."
Among circuits which are suitable for location on a magnetic head carrier is that shown in IBM Technical Disclosure Bulletin, Vol. 16, No. 11, April, 1974, "Read/Write Switch with Remote Write Driver." This circuit has relatively complex, separate write driver circuitry and preamplifier circuitry. Accordingly, it requires a substantial amount of "real estate" on the carrier to accomplish the necessary functions. Another type of preamplifier circuit is shown in IBM TDB, Vol. 16, No. 5, October 1973. U.S. Pat. No. 4,085,383 discloses a diode selection circuit including a preamplifier suitable for low impedance heads.